A orthogonal, curvilinear grid system for World Ocean modelling is examined. It involves the coupling of two non-singular spherical sub-grids, avoiding the North Pole singularity of the standard, spherical coordinates. The two sub-grids are connected in the equatorial Atlantic. It is shown how to minimize the sudden variation in the grid size across the connection line. It is suggested that the two spherical coordinate systems be considered as a single orthogonal, curvilinear coordinate system, in which the metric coefficients and the OGCM governing equations are established.